1. Field of the Invention
The invention relates to molybdenum disulfide catalysts. More particularly, it relates to the preparation of such catalysts having enhanced catalytic properties.
2. Description of the Prior Art
The catalytic hydrogenation of carbon monoxide to form methane is a well known, established methanation reaction. This reaction: EQU CO+3H.sub.2 .fwdarw.CH.sub.4 +H.sub.2 O, (1)
utilizes a synthesis gas, as from the gasification of coal with oxygen and steam. Prior to methanation, the gas stream is commonly treated to provide a desired H.sub.2 /CO ratio and to remove excess CO.sub.2 and deleterious impurities such as sulfur impurities. As the H.sub.2 /CO ratio of the raw synthesis gas is generally substantially below the necessary minimum ratio of 3/1, at least a portion of the carbon monoxide is generally first reacted with steam, over an iron or other suitable catalyst in the well-known "water gas shift" reaction as follows: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2. (2)
Excessive CO.sub.2 in the gas stream is removed by conventional means, such as by treatment with alkaline absorbents. Sulfur impurities are also removed to substantially under 5 ppm, e.g. to less than about 1 ppm, preferably to less than 0.2 ppm, to protect the methanation catalyst from poisoning by such sulfur impurities. Hydrogen sulfide or other sulfur bearing gases are absorbed, selectively or non-selectively, by the absorbent employed for carbon dioxide removal. When necessary, final cleanup may be accomplished by passing the gas stream through iron oxide, zinc oxide or activated carbon to remove residual traces of H.sub.2 S or organic sulfides.
In view of the diminishing supply of natural gas, such methanation techniques are of considerable interest in the art as a means for producing substitute natural gas (SNG) from coal, shale oil, tar sands, petroleum residues, biomass, industrial and municipal waste, and other complex carbonaceous material. While a variety of specific processing techniques for SNG production have been proposed in the art, essentially all of these techniques provide for the steps of (1) gasification, to produce crude mixtures of CO, H.sub.2, CO.sub.2, H.sub.2 O, CH.sub.4 and other trace components; (2) catalytic water gas shift to adjust the CO:H.sub.2 ratio as indicated above; (3) and catalytic methanation in accordance with reaction (1) above and related reactions that might occur, such as: EQU CO.sub.2 +4H.sub.2 .fwdarw.CH.sub.4 +2H.sub.2 O and/or (3) EQU 2CO+2H.sub.2 .fwdarw.CH.sub.4 +CO.sub.2. (4)
The methanation catalysts currently being seriously considered for commercialization are based on nickel or cobalt as the active ingredient. These metallic catalysts are very active, selective and relatively cheap. They are, however, extremely sensitive to poisoning by sulfur compounds. Since almost all of the carbonaceous feeds employed for synthesis gas production contain sulfur that is converted largely to H.sub.2 S during the initial gasification step, costly acid gas purification operations must be included in SNG process designs so as to lower the H.sub.2 S level to the fractional ppm level indicated above to achieve commercially feasible, long catalyst life. It would be highly desirable in the art, therefore, if sulfur-resistant methanation catalysts were commercially available as this would permit a considerable reduction in the degree of gas purification processing required prior to the methanation step in SNG production operations. If such a catalyst would also catalyze water shift reaction (2) effectively, the number of individual processing steps, and the overall cost of SNG production could be even further reduced.
It has long been recognized in the art that molybdenum sulfide, MoS.sub.2, and tungsten sulfide, WS.sub.2, as well as more complex mixed sulfides, are sulfur-tolerant methanation catalysts. MoS.sub.2 occurs native as molybdenite and can be prepared artifically by heating molybdenum dioxide, molybdenum trioxide or ammonium molybdate in H.sub.2 S or sulfur vapor. Thus, Mills and Steffgen, in Catalyst Rev. 8, 159 (1973), review the results of several studies with molybdenum and tungsten sulfide methanation catalysts prepared in a variety of ways. Even the best of these catalysts were only moderately active. In the Stewart patent, U.S. Pat. No. 2,490,488, MoS.sub.2 catalysts modified by the addition of alkali metal compounds are disclosed as shifting the hydrocarbon synthesis of synthesis gas from methane to a mixture of higher molecular weight products. A CO conversion of 95% was achieved at 280.degree. C. and 200 psig, at a commercially impractical space velocity (SV) of 86 hr.sup.-1. A temperature of 410.degree. C. was required to achieve 98% conversion at an SV of 100 hr.sup.-1.
Methanation activity for molybdenum catalysts, including those prepared as sulfides, was reported by Schultz et al, U.S. Bureau of Mines, Rep. Invest. No. 6974 (1967). In the preparation of catalyst L 6135, H.sub.2 S gas and an aqueous solution of aluminum nitrate were added to an ammoniacal aqueous solution of ammonium molybdate to precipitate a mixture of ammonium thiomolybdate and hydrated aluminum hydroxide. This coprecipitate was reduced in H.sub.2 before use. When employed with a stream having a CO:H.sub.2 ratio of 1:3 at 400.degree. C., the CO conversion was 47.6% at a space velocity of only 295 hr.sup.-1. Shultz et al also prepared catalysts by impregnating silica-alumina or activated carbon supports with ammonium molybdate, followed by calcining, to give a supported molybdenum oxide for which a conversion of 76.6% was reported at 420.degree. C. and 21 atm. This catalyst was not sulfided. Other catalysts prepared as oxides by coprecipitating aluminum and molybdate salts, without sulfiding, provided methanation performance similar to that of impregnated materials.
Such previously available molybdenum methanation catalysts, including MoS.sub.2 catalyst materials, are relatively inactive, and are not generally considered to possess sufficient activity to justify use in commercial operations. Despite the desirable sulfur resistant properties of MoS.sub.2 materials, therefore, such available materials have not been suitable for practical use in providing synthetic natural gas to meet existing and anticipated requirements for low-cost, high BTU gaseous heating fuels.
There remains a need in the art, therefore, for an improved methanation catalyst having an acceptable degree of activity for use in commercial operations, coupled with an absence of the extreme sensitivity to poisoning by sulfur compounds that is characteristic of the active nickel and cobalt catalyst compositions. The satisfactory catalytic activity and the reduced acid gas purification requirements thus achieved would enable the overall SNG production operations to be carried out in a manner enhancing, on an overall technical-economic basis, the production of low-cost, high purity SNG as a replacement for natural gas.
It is an object of the invention, therefore, to provide an improved, sulfur-resistant methanation catalyst.
It is another object of the invention to provide a sulfur-resistant molybdenum disulfide catalyst of improved catalytic activity.
It is another object of the invention to provide an improved molybdenum disulfide catalyst capable of enhancing the overall operation for the production of SNG.
With these and other objects in mind, the invention is hereinafter disclosed in detail, the novel features thereof being particularly pointed out in the appended claims.